Copper is the third most widely used metal in the world following iron and aluminum. In 1995, nearly 2 million metric tons of copper were produced from over 40 mines operating in the U.S., which was worth over $5.7 billion. This represents about 20% of the world production of this metal in 1995. The U.S. is the second largest producer of copper, closely followed by Chile. In addition, 0.37 million metric tons of copper were produced from scrap in the U.S. in the same year. Copper is an important element which is used in building construction, 42%; electric and electronic products, 22%; industrial machinery and equipment, 13%; transportation equipment, 13%; and, consumer and general products, 10%.
The production of copper currently heavily relies upon a high temperature technology where copper concentrates, primarily copper sulfides, are subject to roasting and smelting followed by electrowinning and/or solvent extraction to produce elemental copper. This high temperature smelting process has an environmental drawback in which the process stack gas contains sulfur containing components which have to be removed before the gas is allowed to escape to the environment. Furthermore, the process is inherently expensive due to its high energy use and requires expensive maintenance of the smelting operation. In spite of such drawbacks in smelting operation, it is believed that about 70-80% of copper produced in the U.S. is currently carried out by this smelting technology. Only 20 to 30% of copper is recovered hydrometallurgically from the low grade ores by heap leaching where sulfuric acid and air are used.
The applicability of the heap leaching technology to sulfide concentrates which represent the majority of copper produced in this country is doubtful. There is an urgent need to develop a process which is able to treat copper concentrates in both environmentally and economically acceptable ways.
It should be noted, however, that numerous hydrometallurgical methods have been suggested and attempted, some of which have been fairly successful even at the pilot plant level. Vizsolyi et al. ("Copper and Elemental Sulphur from Chalcopyrite by Pressure Leaching," Journal of Metals, pp. 52-59, November 1967) have shown that 98% recovery of copper from chalcopyrite (CuFeS.sub.2), one of the major copper containing minerals, was possible by a pressure leaching. Such a recovery of copper can be obtained at about 120.degree. C. and 500 psi pressure after a 2.5 hour leaching with 325 mesh size mineral particles. Although the recovery was good, the pressure was too excessive and furthermore, the treatment of the iron produced in such a system is usually proven to be too costly.
U.S. Pat. No. 5,096,486, to Anderson et al., and U.S. Pat. No. 4,084,961, to Caldon et al., have shown that copper sulfides can be treated to recover copper using sulfuric acid and sodium nitrite or nitric acid at 70.degree. to 160.degree. C. after 0.5 to 4 hours of leaching. When nitric acid was used the temperature could be below 100.degree. C. The copper recovery was about 90-95%.
U.S. Pat. Nos. 4,132,758 and 4,144,310 to Frankiewicz et al. have shown that copper sulfides could be treated using nitrogen dioxide (NO.sub.2) at 100.degree. C. for about 6 hours of leaching. The copper recovery was about 90%. These technologies in general induce a very corrosive environment and hence the maintenance of the leaching reactor causes an industrial nightmare. The treatment of dissolved iron in the liquid is also costly.
Many researchers have used halogen salts such as chlorine/chloride and bromide (U.S. Pat. No. 4,039,406 to Stanley et al., and J. E. Dutrizac, "The Leaching of Sulphide Minerals in Chloride," Hydrometallurgy, vol. 29 (1992), pp. 1-45) to recover copper from various copper-bearing minerals. The process taught by Stanley et al. utilizes chloride or bromide to convert copper sulfides to insoluble copper sulfate. These technologies are in general very slow in the rate of copper recovery and there are many problems associated with the removal or iron and effective recovery of copper from the leach liquor. As a result, none of these processes are being used in the copper producing industry.
Researchers at the South Dakota School of Mines and Technology (SDSM&T) have recently developed noble and environmentally friendly technologies of extracting precious metals, including platinum group metals, silver and gold, from ores and automobile catalytic converters using ammonia and/or halogen salts (U.S. Pat. Nos. 5,114,687, 5,308,381, 5,328,669 and 5,542,957 all herein incorporated by reference). These processes teach how well precious and other metals could be recovered from ores and other materials, and also demonstrate how such metals could be recovered using environmentally benign technologies.
Although environmentally friendly technologies for extracting precious metals has been developed, a need still exists to provide more environmentally friendly methods to recover copper, particularly copper in the form of copper sulfides.
The present invention solves this need by providing an improved process for the extraction of copper from various source materials which is both economical and environmentally friendly.